Electrodeposition of rhodium



PatentedOct. 13, 1936 2,057,475 so ELECTRODEPOSITION or anonnnu "Sigmund Colin, New York, N. Y., assignor to Newark, N. 3., a cor- Baker & Company, Inc poration of New Jersey No Drawing.

a Serial No.

Application March 31 6 Claims. (01.29491) This invention relates to a process for the electrolytic deposition of rhodium and to a bath for effecting said deposition.

The precious metal, rhodium, according to general practice is usually plated from strongly acid aqueous baths containing said metal in the form of salts of strong mineral acids, the sulfate bath being that most usually employed. This, form of bath in order to be at all effective contains ahigh concentration of free sulfuric acid with the result, that during electrol "ysis a spray of this acid is given off thereby rendering the plating operation a possible source of danger to the plater. In addition, said acid spray may attachbase metal objects adjacent to the plating tank. While this dimculty could be overcome by providing a hood over the plating tank, the cost incident to thisconstruction which must be acid proof is a drawback to this process. Still another drawback to the use of' the above bath resides in the corrosive action of the free acid on base metal cathodes which must not therefore be immersed or left stand- ;ing in the bath unless the current is on. Furthermore, the deposit obtained with this type of bath is not always bright and coherent and in order to obtain any satisfactory deposit, very careful regulation of the plating conditions must be maintained.

Since the cost of the above precious metal and its salts is very high, it can, be appreciated that in view of the many objections to the use of the above bath for plating these metals, the plating of said metal on a commercial scale has its i 35 hardly obtained any degree of importance.

Lhave now found that the many objections to the plating of said precious metal in the strongly acid bath above mentioned, can be overcome and clear, brilliant deposits of said metal obtained without a careful manipulation of the plating process, by depositing said metal from aqueous baths containing saidmetal in the form of salts with weak acids.

it is accordingly an object of this invention to provide a process for obtaining clear, brilliant deposits of rhodium without a careful and expensive control of the plating operation and without subjecting the operator to dangerous 50 and disagreeable working conditions.

It is a further object of this invention to provide a simple, efficient, relatively inexpensive process for electrolytically obtaining rhodium in dense deposits of a high clarity and brilliance.

Other and further important objects of this T OFFICE invention will become apparent from the following description and appended claims.

The plating baths with which my process is operative, as stated above, comprise essentially aqueous bathscontaining the precious metal rhodium in the form of salts of weak acids. Any weak acid, whether organic or inorganic, which forms a salt with the above metal may be employed in connection with said metal for furnishing the metallic ions to the electrolyte. These weak acids will comprise preferably acids of an acidity of that of tartaric, oxalic, citric,

benzoic, lactic and-like organic acids and also phosphoric and similar weak inorganic acids.

Moreover, said acids may be combined with said metal either as simple salts or as complex salts with the alkali metals and ammonium as for example rhodium ammonium oxalate, rhodium ammonium citrate, rhodium ammonium tartrate, rhodium ammonium benzoate, rhodium ammoniumlactate and the like.

A peculiarity of these baths which greatly facilitates the deposition of the metal therefrom and enhances the emciency of the electrolysis is the fact that they may be employed with great success either in an acid, neutral or alkaline condition. The proper pH oi. the bath, if it is to be alkaline, may be provided by adding a certain amount of a weak or strong alkali to an aqueous solution of the simple or double salts of said precious metal. bath on the other hand may be controlled by The acidity of the electrical resistance.

The cathode of course may be any metal upon which it is desired .3 plate, such as silver, gold, platinum, copper, brass, German silver, pewter and the like. 01 course, the cathode, as in general plating should be clean and smooth to facilitate the production of clear, dense, brilliant deposits.

The concentration of the metal ions in solution may vary depending upon-the conditions of temperature, electrical pressure and cathode current density but need not be high to secure good deposits.

As a general rule, the anode current density is not of great importance but should be less than that of the cathode.

It is also not necessary with the baths 0! my invention to agitate the solution during plating or provide any other means to efiect depolarization of the electrodes, although a more even plate may be obtained if this is done.

The following example will serve to more fully illustrate the nature of my invention, but it should be borne in mind that my process is not limited to the specific conditions recited therein.

An aqueous solution of a salt of rhodium with a weak acid such as rhodium oxalate, citrate, tartrate, benzoate, lactate, phosphate or the like or of a double salt of rhodium such as rhodium ammonium oxalate, rhodium ammonium citrate, rhodium ammonium benzoate or the like containing about 1 gram of rhodium per liter of solution is electrolyzed with a. platinum anode and a smooth copper cathode which it is desired to plate at a voltage of about 4 volts and a cathodic current density of about 2 amperes per sq. inch. The electrolyte is maintained at a temperature of between and C. during the electrolysis. In this way dense, brilliant deposits of rhodium are obtained. Of course, the usual D. C. current is used.

As stated above, the electrolyte may be acid, neutral or alkaline which conditions may be obtained by the use of the proper amount of a suitable acid or alkali. Generally an alkaline electrolyte is preferable. If, however, an acid bath is used, it is obvious that the acidity will not be made so high as to cause deposition of hydrogen to the exclusion of the metal. Likewise, excessive alkalinity should be avoided. These are however obvious details which will be varied at the option of the skilled plater under diilerent desired conditions of plating.

It can thus be seen that I have provided a novel and highly efllcient process for obtaining dense, brilliant deposits of rhodium.

Various details of my process may of course be varied and I therefore do not intend to be limited in the scope of my invention except as necessitated by the prior art.

I claim:

1. The process oi depositing a brilliant and dense layer of rhodium for ornamental purposes which comprises passing a D. C. current through an aqueous alkaline bath containing said metal combined with phosphoric acid, at a temperature ranging from 60 to 90 C.

2. The process of plating a brilliant and dense layer of rhodium'for ornamental purposes electrolytically which comprises passing a D. C. current through a solution containing rhodium ammonium phosphate.

3. The process as defined in the preceding claim wherein the bath is maintained at a temperature of 60 to 90 C.

4. The process of electro-plating rhodium which comprises passing a D. C. current at about 4 volts and a cathode current density of about 2 amperes per sq. inch through a rhodium phosphate solution maintained at a temperature of 60 to 90 C.

5. A process as defined in claim 4 wherein the metal is deposited from a solution of rhodium ammonium phosphate containing about 1 gram of rhodium per liter of solution.

6. A process of electro-depositing rhodium which comprises passing a D. C. current of about 4 volts at a cathodic current density of 2 amperes per sq. inch from a platinum anode through an alkaline rhodium ammonium phosphate solution containing about 1 gram of rhodium per liter of solution.

SIGMUND COHN. 

